This is an old SFBC volume, published in 1966. It actually holds up rather well. It puts me in mind of an early James Bond with an SF twist. Just about everyone in the world takes a happy pill. Most people are just - well, happy - but a few become 'sensitives' or 'specials'. Our hero is a sensitive working for the organisation that has cornered the market in the  drug. The overall plot is a little predictable by today's standards, but given that it's  well over 50 years old, that's not altogether surprising.
  This isn't a book you are likely to see anywhere but in a second-hand shop somewhere, but if you do, I can quite recommend it.